Because of thermal expansion problems, most large sized steam generators are supported from the top, being hung from a rigid girder assembly. The structurally large girder assembly necessary for supporting a steam generator is quite expensive, and also requires relatively high head room, making access difficult to some parts of the assembly. Thus, some recently constructed steam generators have been built which are supported from the bottom. These have all been oil or gas fired units, where it is possible to have each of the tube-lined walls of the furnace extending upwardly in a single plate, so as to form a part of the load supporting structure. In a coal fired unit, it is necessary to have a hopper bottom, through which the coal ash and/or slag can be discharged during operation of the unit without too much loss of heat by radiation through the bottom opening.